Sex change and effective population size: implications for population genetic studies in marine fish

Abstract

Large variance in reproductive success is the primary factor that reduces effective population size (Ne) in natural populations. In sequentially hermaphroditic (sex-changing) fish, the sex ratio is typically skewed and biased towards the 'first' sex, while reproductive success increases considerably after sex change. Therefore, sex-changing fish populations are theoretically expected to have lower Ne than gonochorists (separate sexes), assuming all other parameters are essentially equal. In this study, we estimate Ne from genetic data collected from two ecologically similar species living along the eastern coast of South Africa: one gonochoristic, the 'santer' sea bream Cheimerius nufar, and one protogynous (female-first) sex changer, the 'slinger' sea bream Chrysoblephus puniceus. For both species, no evidence of genetic structuring, nor significant variation in genetic diversity, was found in the study area. Estimates of contemporary Ne were significantly lower in the protogynous species, but the same pattern was not apparent over historical timescales. Overall, our results show that sequential hermaphroditism may affect Ne differently over varying time frames, and that demographic signatures inferred from genetic markers with different inheritance modes also need to be interpreted cautiously, in relation to sex-changing life histories.

Figure 1

Map: sampling locations off the KwaZulu-Natal Coast of South Africa.

Figure 2

Effective population size estimates: effective population size of Cheimerius nufar (santer, in black) and Chrysoblephus puniceus (slinger, in grey) estimated with different allelic frequencies (or P_crit). The three sampled populations were pooled.

Figure 3

Historical female effective population size: historical female effective population size (N_ef) of slinger Chrysoblephus puniceus (continuous line) and santer Cheimerius nufar (dotted line) based on θ for different mutation rates (μ) and three generation lengths (T=3, 5 and 8 years). Red values correspond to N_ef estimations for μ=11% and T=5. A full color version of this figure is available at the Heredity journal online.

Figure 4

Bayesian Skyline plots: Bayesian Skyline plots for santer Cheimerius nufar and slinger Chrysoblephus puniceus. The continuous and dashed lines represent the median and mean values, respectively.